NiMH battery charging problem

I have a battery (8 cells in series, NiMH AA cells (1.2 V to 1.3 V), 1300 mAh per cell).

Cells used-

Total low voltage = 1.2*8 = 9.6 V

Total high voltage = 1.3*8 = 10.4 V

I have a circuit that can give a constant 12 V, 1 A output. When I measure the voltage before charging starts it shows 9.6-9.7 V and as soon as I connect the 12 V source the voltage jumps to 10.1 V and very quickly reaches 10.3 V and 10.4 V (within 2-3 minutes). Even after disconnecting the 12 V source the battery voltage remains at 10.4 V.

Current flowing is about 500 mA and reduces as voltage increases.

Q. I can't understand how the battery is charging so quickly?

I have made cut-off circuitry that measures battery voltage and stops overcharging (cut-off point is 10.3 V). Right now as soon as I connect 12 V it cuts off as voltage increases rapidly.

Is it possible that the 12 V is getting added with the battery voltage and that is why a sudden increase in voltage is seen? After I disconnect the 12 V source a drop of about 0.4 V is seen.

What should I do to properly charge a NiMH battery?

Below is the schematic I am using. It is AC/DC input and a 12 V, 1 A DC output.

Here is the datasheet and schematic link.

• That's not a proper battery charger - the correct way, at minimum, needs to monitor current as well. A safe one will include thermal protection. Your circuit is only monitoring voltage. It's essentially a voltage supply. You should get a proper battery charger. Jan 26 at 19:19